1. Field of the Invention
The present invention relates to a crank assembly for an electrical bicycle and, more particularly, to a crank assembly which is operated by reciprocatedly pivoting approximately 60 degrees about a crank axle of the bicycle.
2. Brief Description of the Prior Art
FIG. 5 shows a side elevation view of a prior art electrical bicycle which is driven by a motor 81 powered by a set of batteries 80 disposed on a down tube 70 of the bicycle. A crank assembly including two crank arms 60 each with a pedal disposed thereto transversely extends through a bottom bracket of the bicycle so that a rider rotating the crank arms 60 cooperated with the motor 81 will ride the bicycle with less effect. Referring to FIG. 6, in order to move the bicycle forward, the crank arms 60 have to be rotated 360 degrees. A suitable length of each one of the crank arms 60 is 170 mm and a minimum distance measured from the lowermost pedal to the ground is 120 mm. When the bicycle is ridden round a corner and tilts accordingly, the bicycle has an inclined angle "A" of 26 degrees regarding to a vertical plane passing through two points where two tires contact the ground. This inclination also limits the length of the crank arm 60 as a longer crank arm will hit the road surface. However, the shorter the crank arms 60 are, the more force should be applied by the rider to rotate them. Furthermore, the location of the set of batteries 80 increases a distance measured from a top of the set of the batteries 80 to the ground so that it would be inconvenient for the rider to move his/her leg across the set of batteries 80 and sit on a seat 82 of the bicycle.
The present invention intends to provide an improved crank assembly for an electrical bicycle to mitigate and/or obviate the above-mentioned problems.